1. Field of the Invention
The present invention relates to a process and apparatus for the treatment of waste materials, and in particular to a process and apparatus for the treatment of incinerator residue produced from commercial, industrial and municipal refuse, and to an aggregate material produced by that process
2. Background of the Invention
The problems associated with disposing of waste materials in an environmentally sound manner are well known. One of the serious concerns our consumption-oriented modern industrial society faces is what to do with municipal refuse. Traditional solutions, especially disposal of unprocessed refuse in landfills, are rapidly losing their viability, as acceptable disposal sites become increasingly difficult to find, and as such sites have become recognized as sources of unacceptable pollution of their environs. See, e.g., H. A. Neal et al., Solid Waste Management and the Environment: The Mounting Garbage and Trash Crisis (Prentice-Hall, Inc. Englewood Cliffs, N.J. 1987).
While raw municipal solid waste contains many materials that can be separated and recycled, there is always a residuum which must be somehow disposed of. One general approach to municipal waste disposal has been incineration, often now combined with cogeneration of power such as in modern "trash-to-steam" facilities. Municipal refuse incinerators must include means for minimizing the emission of environmentally harmful substances, such as HCl, SO.sub.2, NO.sub.x, polynuclear aromatic compounds, and dioxin. In addition, ash is a major product of municipal trash incinerators which itself must be disposed of in an environmentally acceptable manner. Two types of residue are produced by incineration: low density fly ash which must be removed by filtration, electrostatic precipitation, or a similar technique from the stack exhaust, and bottom "ash," a residue which is too dense to be carried with the hot exhaust gases.
Incineration may concentrate toxic materials present at low levels in the raw refuse so that the ash produced contains environmentally unacceptable proportions of pollutants such as mercury and cadmium. If untreated incinerator ash is disposed of in a landfill, such toxic material may leach out and seriously contaminate groundwater in the vicinity of the landfill. Clearly, there is a need for a process for treating incinerator ash to reduce the likelihood that toxic materials present in the ash, such as "heavy metals" including mercury and Cadmium, will leach out to contaminate local groundwater in the landfill area.
Many processes have been proposed for the incineration of municipal wastes and treatment of the residues formed thereby. For example, it has been suggested that fly ash produced by incinerating municipal waste be treated with a slightly acid medium to remove soluble materials, that the soluble contaminants removed thereby be treated chemically to precipitate them, and that the precipitate be immobilized as a compact material. H. Vogg, Inter. Chem. Eng., vol. 27, no. 2 (April 1987) pp. 177-182. At the same time it has been advised that the incinerator itself be operated to produce an inert slag which can be reused. Id.
Hot bottom "ash" from a municipal refuse incinerator may be first treated by quenching with water in a quench tank. See, e.g., K. R. Lauer et al., "Profitable Utilization of Incinerator Residue From Municipal Waste" Proc. Fifth Mineral Waste Utilization Symp. (E. Aleshin, ed. Chicago 1976) pp. 215-218. The wet quenched bottom ash can be dried, screened to remove bulk metal, and separated into magnetic and non-magnetic fractions using a magnetic separator. The fly ash can be disposed of separately, such as by use as a supplement for cement, or it can be combined with the residue in the quench tank. Id. The non-magnetic fraction can be used as an aggregate for subbases or base courses (such as for highway construction or the like), as an aggregate for Portland cement concrete and articles formed therefrom such as concrete masonry blocks, or as an aggregate for bituminous concrete. The non-magnetic fraction includes glass, sand, slag, ash and some magnetic materials. The use of this fraction as an aggregate in Portland cement concrete is complicated by a potential alkali-glass reaction which can cause concrete expansion detrimental to the concrete. Id.
The non-magnetic fraction can be further processed. For example, the glass fraction can be separated and used to produce structural materials such as bricks. The waste glass can be ground, mixed with clay and sodium silicate, pelletized and sintered to give expanded glass aggregate pellets useful as aggregate for Portland cement concrete. K. J. Liles, "Lightweight Structural Concrete Aggregate From Municipal Waste Materials," Proc. Fifth Mineral Waste Utilization Symp. (E. Aleshin, ed. Chicago 1976) pp. 219-222.
Fine particulate waste can be agglomerated by adding water or a binder liquid to the dry waste powder in a disk or barrel pelletizer, the resulting green pellets being subsequently fired in a rotary kiln or traveling grate sintering machine to produce aggregate for concrete. E. Aleshin et al., "Aggregates Manufactured From Waste Materials," Living With Marginal Aggregates (ASTM Special Tech. Pub. 597, C. H. Best, ed. ASTM Philadelphia 1975) pp. 85-96. The rotary kiln can produce a lightweight expanded aggregate having a fused surface, having low water absorption and suitable for cast-in-place concrete. The traveling grate sintering machine produces a more porous aggregate which can be used in concrete blocks. Many types of mineral wastes, such as flyash from coal-burning power plants, talconite tailings, phosphate waste, and the like can be used in this manner. Id.
U.S. Pat. No. 3,383,228 discloses a process and apparatus for subjecting waste, such as municipal waste, to destructive distillation and for subsequent compaction of ash produced. A binder can be used to produce cohesive blocks of ash.
Pulverized municipal refuse can be used to fire a cement kiln, the resulting ash being incorporated into the cement clinker, according to U.S. Pat. No. 4,022,630.
U.S. Pat. No. 3,734,988 relates to a process for converting municipal refuse into useful building products such as blocks, bricks and wall-board. The average size of the waste is reduced by shredding, and the shredded waste is decomposed in the presence of nitrogen, water and air. The decomposed product is dried to render it biologically inert and ground to a fine powder. The fine powder is mixed with a binder such as a hydraulic setting cement and molded into a useful shape and cured until a specified strength is achieved. The cured, molded products can be coated to provide desired architectural characteristics.
U.S. Pat. No. 4,744,829 discloses a method of producing agglomerates which are resistant to leaching out from finely divided waste materials. A hydraulic agent such as lime or Portland cement is dissolved in water under forced mixing conditions until a colloidal solution is obtained. The finely divided waste is mixed with the colloidal solution and agglomerated, as with a pelletizer, and cured. When calcium hydroxide is used as the binder, cure can be accelerated by application of CO.sub.2.
U.S. Pat. No. 4,629,509 discloses that cadmium and lead, which are toxic metals having relatively high volatility, tend to accumulate in fly ash resulting from burning municipal refuse. While the toxicity of the fly ash can be reduced somewhat by mixing it with bottom ash, the resulting mixture can exhibit toxic metal levels above standards set by the U.S. Environmental Protection Agency. If the mixture is used as a landfill material, the toxic metals can leach out to contaminate groundwater. However, the lead and cadmium in the residue obtained from refuse incineration can be immobilized by mixing the residue with either calcium sulfide or a mixture of dry lime and an inorganic sulfide.
U.S. Pat. No. 4,226,631 discloses a method of solidifying toxic wastes, such as harbor dredging sludges and sludges discharged from chemical industry, by use of a mixture of a lipid, such as a fatty oil or higher fatty acid, and a hydraulic cement or cement-type solidifying agent. The use of the lipid substantially reduces the concentration of certain toxic leachates.
U.S. Pat. No. 4,209,335 discloses the use of a mixture of a sulfate and alkaline metal salt as an additive for a hydraulic cement binder used to solidify toxic wastes, such as scrubber sludges and "community waste" incinerated ash. Noting that some wastes contain substances, such as certain heavy metal element compounds, which inhibit hydraulic cement from hardening by hydration, the inventors provide for the addition of a third additive component, an alkaline earth metal oxide or hydroxide, such as calcium oxide, which can be supplied by added fly ash. Example 5 reports the effect of using the additive in admixtures of Portland cement and electrostatic precipitator dust (fly ash) discharged from a municipal incinerator and containing considerable amounts of heavy metal elements which are known to hamper hydraulic cement from hardening by hydration. The additive provides higher compressive strengths than were otherwise observed. Example 6 reports the extent to which toxic metals, including mercury, cadmium, lead, hexavalent chromium, and arsenic are leached from toxic wastes solidified with additive-containing Portland cement. Toxic metal leachate concentrations are reduced when the additive is used in Portland cement to solidify either fly ash or municipal refuse incinerator ash.
U.S. Pat. No. 3,980,558 relates to a process for disposing of semi-liquid wastes containing soluble toxic materials to substantially prevent leaching of these toxic materials. A hydraulic cement such as Portland cement is admixed with the waste to provide a fluid mass which will set up to a rock-like consistency. An inert granular filler or aggregate for the cement, such as clay, bentonite, kaolin or the like can also be added.
U.S. Pat. No. 4,669,397 relates to a process for recovering useful materials from refuse fuel ash. Bottom ash from a refuse incinerator is fed to a magnetic separator to separate the major part of ferrous materials contained therein and subsequently separated into oversize, midsize, and undersize fractions. The undersize fraction is milled and subjected to another magnetic separation step, with the nonmagnetic fraction being separated into undersize and oversize materials. The oversize from this step are conveyed to a nonferrous metal collection operation, while the undersize from both the initial and the second separation steps are collected as aggregate. This aggregate can be used to dilute fly ash to reduce the concentration of toxic materials contained therein so that the so diluted flyash can be disposed of in a conventional landfill.
Portland cement has been used to trap a wide variety of toxic wastes in a solid matrix; including arsenic (U.S. Pat. Nos. 4,329,179; 4,142,912; 4,046,674; 3,933,624; 3,804,750), radwaste (U.S. Pat. Nos. 3,988,258; 4,122,028; 4,017,417; 4,504,317; 4,530,723; 4,533,395), fossil fuel ash (U.S. Pat. No. 4,726,710) acidic liquors containing heavy metals, such as from electroplating and metal finishing (U.S. Pat. No. 4,741,776), washing water filter sludge (U.S. Pat. No. 4,304,674), and oxysludge from steel manufacture (U.S. Pat. No. 4,601,832).
U.S. Pat. No. 4,331,088 discloses the use of a vertical shaft furnace to slag-encapsulate hazardous chemical wastes.
U.S. Pat. No. 4,615,809 relates to a method of stabilizing organic sludges with Portland cement, fly ash, gypsum and lime, the Portland cement being added last to enhance its function as a binder.
U.S. Pat. No. 4,116,705 relates to a method of encapsulating hazardous waste in a crystalline matrix binder formed by a slurry of Portland cement and aluminum silicate or aluminosilicate (e.g., vermiculite or fly ash).
U.S. Pat. No. 4,350,620 relates to a process for filtering radioactive particles from a fluid medium employing a filter aid comprising fibrids formed from polyethylene and/or polypropylene. The filter cake can be extruded using conventional plastics processing equipment to give plastic spheres in which radioactive particles are encapsulated.
U.S. Pat. No. 4,800,024 discloses a method of treating water containing heavy metals or radioactive metal using a water-insoluble carboxylated cellulose-transition metal oxide mixture to precipitate the target metal(s). The resulting precipitate can then be dried and calcined to give a non-leaching, ceramic-type spinel, or admixed with a suitable leach-resistant matrix material, such as asphalt or grout.
U.S. Pat. No. 4,623,459 relates to the use of aqueous asphaltic emulsions to tie up liquid and semi-liquid wastes in a hydrophobic, leach-resistant mass.
U.S. Pat. No. 4,741,834 discloses the use of Type C fly ash (from subbituminous or lignite coals) as a rectifying agent in land reclamation operations.
Despite the progress which has been made toward addressing the problems posed by the disposal of solid wastes from municipal, commercial and industrial sources, there remains a substantial need for an inexpensive, reliable method for treating incinerated waste, which often contains significant amounts of toxic materials, such as heavy metals, so that such incinerated waste can be disposed of in an environmentally sound manner.
While the incineration of waste materials is becoming an increasing popular response to the disposal problem, there has already been a substantial capital investment in incineration facilities by municipal authorities and by industry. Thus, there is a need for an incinerated waste treatment process and for apparatus for carrying out this process which build on this past capital investment and which can also be used in future incineration facilities.
In addition, there is a need for an on-site treatment process and on-site treatment apparatus which can be used to treat incineration residue as it is discharged from the incinerator, so that the costs and hazards associated with the shipping and storage of untreated residue are minimized or eliminated. Further, there is a need for a treatment process and apparatus which can be used in either a batch or continuous mode to treat incinerator residues. Similarly, there is a need for a process which can treat incinerator residues to provide a useful construction material, the size and strength of which can be controlled in the production process.